There Are About 4,700 Asteroids Close Enough To Smash Into Earth

This diagram shows an edge-on view of our solar system. The dots represent a snapshot of the population of near-Earth asteroids (blue) and potentially hazardous asteroids (bright orange) that scientists think are likely to exist based on the new survey.

According to the space agency, potentially hazardous asteroids, or PHAs, are large enough to pass through Earth's atmosphere without being burnt to a crisp and cause widespread damage.

These space rocks are bigger than 330 feet across (that's slightly less than the length of a football field ) and come within 5 million miles of Earth.

NASAThis diagram illustrates the differences between orbits of a typical near-Earth asteroid (blue) and a potentially hazardous asteroid (orange). PHAs are a subset of the near-Earth asteroids (NEAs). They have the closest orbits to Earth's orbit and are large enough to survive passage through Earth's atmosphere.

About 20 to 30 percent of the 4,700 PHAs have been found, NASA says.

The observations were made by the agency's WISE spacecraft, which scans the sky with infrared light to detect the heat of asteroids. Unlike your run-of-the-mill telescope where different size asteroids appear as uniform tiny dots, WISE is able to sense the difference between objects that are small and very reflective and objects that are large and very dark. This helps astronomers determine the composition and size of asteroids, which is key to assessing their potential risk to Earth.

The new figure suggests that about twice as many PHAs are closely aligned with the plane of Earth's orbit than previously thought. Asteroids with "lower-inclination" orbits have two implications: they would be more likely to approach our planet, but are easier for spacecraft to reach for future exploration missions.